1. Field of the Invention
The present invention relates to an LED (light emitting diode) array device for use, for example, in a line printing device to form a latent image on a photoconductive surface. It also relates to a method for making such an LED array device.
2. Description of the Prior Art
The LED array device is generally defined by a plurality of monolithic LED array units, each unit containing a plurality of LED elements, such as 64 elements, aligned straight with a predetermined pitch, such as 1/8 millimeter to 1/16 millimeter. The plurality of LED units are aligned to form an LED array device. Such an LED array device is disclosed, for example, in U.S. Pat. No. 3,850,517 to Stephany et al. issued Nov. 26, 1974, or in Proceedings of the SID, Vol. 23/2, 1982, pages 81 through 84, and entitled "Optical Printer Using LED Array", by K. Tateishi et al.
Referring to FIG. 1, LED elements 2, according to the prior art, are formed on a wafer 1 aligned in two orthogonal directions, and are particularly aligned in a predetermined pitch d1 in the horizontal direction. The light emission of an LED element 2 is actually effected at the boundary of the P region 11 where a PN junction is formed, as best shown in FIG. 2. Extending from each LED element 2 is an electrode 3 for the electric connection thereto. The wafer 1 is cut into chips along lines 4 and 5. The obtained chips 6 define LED array units, each having a size of, for example, 1.times.8 square millimeters. The cutting of wafer 1 is generally done by the steps of scribing the wafer surface along lines 4 and 5 which are parallel to the cleavage and applying pressure to break the wafer into chips. This method of cutting often results in a burr 7, such as shown in FIG. 2. The burr becomes greater as the thickness of the base 10 becomes bigger.
When LED units 6 so obtained are aligned to define an LED array device, burrs 7 results in undesirable spacing at a joint between jacent LED units 6, causing in a long pitch d2 between LED elements 2 on two LED units.
To solve the above problem, a number of arrangements for the LED array device have been proposed, and one of which is disclosed in U.S. Pat. No. 4,318,597 to Kotani et al. issued March 9, 1982. According to this patent, LED units are aligned not straight but in a zig-zag form. Another arrangement is disclosed in Japanese Utility Model Application Laid-Open Publication (Jikkaisho) No. 57-130451. According to this publication, LED units are aligned straight when viewed from the front, but alternately in two different levels, high and low. Accordingly, the jacent LED units do not touch each other.
According to the prior art arrangements described above, the positioning and holding the LED units at the proper positions are very difficult. Moreover, such a difficulty further results in the difficulty in the adjustment in the optical system.